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  <title>Kaleidoscope: Extending the Language: Control Flow</title>
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  <meta name="author" content="Chris Lattner">
  <meta name="author" content="Max Shawabkeh">
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<div class="doc_title">Kaleidoscope: Extending the Language: Control Flow</div>

<ul>
<li>
  <a href="http://www.llvm.org/docs/tutorial/index.html">
    Up to Tutorial Index
  </a>
</li>
<li>Chapter 5
  <ol>
    <li><a href="#intro">Chapter 5 Introduction</a></li>
    <li><a href="#ifthen">If/Then/Else</a>
    <ol>
      <li><a href="#iflexer">Lexer Extensions</a></li>
      <li><a href="#ifast">AST Extensions</a></li>
      <li><a href="#ifparser">Parser Extensions</a></li>
      <li><a href="#ifir">LLVM IR</a></li>
      <li><a href="#ifcodegen">Code Generation</a></li>
    </ol>
    </li>
    <li><a href="#for">'for' Loop Expression</a>
    <ol>
      <li><a href="#forlexer">Lexer Extensions</a></li>
      <li><a href="#forast">AST Extensions</a></li>
      <li><a href="#forparser">Parser Extensions</a></li>
      <li><a href="#forir">LLVM IR</a></li>
      <li><a href="#forcodegen">Code Generation</a></li>
    </ol>
    </li>
    <li><a href="#code">Full Code Listing</a></li>
  </ol>
</li>
<li><a href="PythonLangImpl6.html">Chapter 6</a>: Extending the Language:
User-defined Operators</li>
</ul>

<div class="doc_author">
  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
    and <a href="http://max99x.com">Max Shawabkeh</a>
  </p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro">Chapter 5 Introduction</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Welcome to Chapter 5 of the
"<a href="http://www.llvm.org/docs/tutorial/index.html">Implementing a language
with LLVM</a>" tutorial.  Parts 1-4 described the implementation of the simple
Kaleidoscope language and included support for generating LLVM IR, followed by
optimizations and a JIT compiler.  Unfortunately, as presented, Kaleidoscope is
mostly useless: it has no control flow other than call and return.  This means
that you can't have conditional branches in the code, significantly limiting its
power.  In this episode of "build that compiler", we'll extend Kaleidoscope to
have an if/then/else expression plus a simple 'for' loop.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="ifthen">If/Then/Else</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
Extending Kaleidoscope to support if/then/else is quite straightforward.  It
basically requires adding lexer support for this "new" concept to the lexer,
parser, AST, and LLVM code emitter.  This example is nice, because it shows how
easy it is to "grow" a language over time, incrementally extending it as new
ideas are discovered.</p>

<p>Before we get going on "how" we add this extension, lets talk about "what" we
want.  The basic idea is that we want to be able to write this sort of thing:
</p>

<div class="doc_code">
<pre>
def fib(x)
  if x &lt; 3 then
    1
  else
    fib(x-1) + fib(x-2)
</pre>
</div>

<p>In Kaleidoscope, every construct is an expression: there are no statements.
As such, the if/then/else expression needs to return a value like any other.
Since we're using a mostly functional form, we'll have it evaluate its
conditional, then return the 'then' or 'else' value based on how the condition
was resolved.  This is very similar to the C "?:" expression.</p>

<p>The semantics of the if/then/else expression is that it evaluates the
condition to a boolean equality value: 0.0 is considered to be false and
everything else is considered to be true.
If the condition is true, the first subexpression is evaluated and returned, if
the condition is false, the second subexpression is evaluated and returned.
Since Kaleidoscope allows side-effects, this behavior is important to nail down.
</p>

<p>Now that we know what we "want", let's break this down into its constituent
pieces.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="iflexer">Lexer Extensions for
If/Then/Else</a></div>
<!-- ======================================================================= -->


<div class="doc_text">

<p>The lexer extensions are straightforward.  First we add new token classes for
the relevant tokens:</p>

<div class="doc_code">
<pre>
class IfToken(object): pass
class ThenToken(object): pass
class ElseToken(object): pass
</pre>
</div>

<p>Once we have that, we recognize the new keywords in the lexer. This is pretty
simple stuff:</p>

<div class="doc_code">
<pre>
      ...
      if identifier == 'def':
        yield DefToken()
      elif identifier == 'extern':
        yield ExternToken()
      <b>elif identifier == 'if':
        yield IfToken()
      elif identifier == 'then':
        yield ThenToken()
      elif identifier == 'else':
        yield ElseToken()</b>
      else:
        yield IdentifierToken(identifier)
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="ifast">AST Extensions for
 If/Then/Else</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>To represent the new expression we add a new AST node for it:</p>

<div class="doc_code">
<pre>
# Expression class for if/then/else.
class IfExpressionNode(ExpressionNode):

  def __init__(self, condition, then_branch, else_branch):
    self.condition = condition
    self.then_branch = then_branch
    self.else_branch = else_branch

  def CodeGen(self):
    ...
</pre>
</div>

<p>The AST node just has pointers to the various subexpressions.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="ifparser">Parser Extensions for
If/Then/Else</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>Now that we have the relevant tokens coming from the lexer and we have the
AST node to build, our parsing logic is relatively straightforward.  First we
define a new parsing function:</p>

<div class="doc_code">
<pre>
  # ifexpr ::= 'if' expression 'then' expression 'else' expression
  def ParseIfExpr(self):
    self.Next()  # eat the if.

    # condition.
    condition = self.ParseExpression()

    if not isinstance(self.current, ThenToken):
      raise RuntimeError('Expected "then".')
    self.Next()  # eat the then.

    then_branch = self.ParseExpression()

    if not isinstance(self.current, ElseToken):
      raise RuntimeError('Expected "else".')
    self.Next()  # eat the else.

    else_branch = self.ParseExpression()

    return IfExpressionNode(condition, then_branch, else_branch)
</pre>
</div>

<p>Next we hook it up as a primary expression:</p>

<div class="doc_code">
<pre>
  def ParsePrimary(self):
    if isinstance(self.current, IdentifierToken):
      return self.ParseIdentifierExpr()
    elif isinstance(self.current, NumberToken):
      return self.ParseNumberExpr();
    <b>elif isinstance(self.current, IfToken):
      return self.ParseIfExpr()</b>
    elif self.current == CharacterToken('('):
      return self.ParseParenExpr()
    else:
      raise RuntimeError('Unknown token when expecting an expression.')
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="ifir">LLVM IR for If/Then/Else</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>Now that we have it parsing and building the AST, the final piece is adding
LLVM code generation support.  This is the most interesting part of the
if/then/else example, because this is where it starts to introduce new concepts.
All of the code above has been thoroughly described in previous chapters.
</p>

<p>To motivate the code we want to produce, lets take a look at a simple
example.  Consider:</p>

<div class="doc_code">
<pre>
extern foo();
extern bar();
def baz(x) if x then foo() else bar();
</pre>
</div>

<p>If you disable optimizations, the code you'll (soon) get from Kaleidoscope
looks something like this:</p>

<div class="doc_code">
<pre>
declare double @foo()

declare double @bar()

define double @baz(double %x) {
entry:
	%ifcond = fcmp one double %x, 0.000000e+00
	br i1 %ifcond, label %then, label %else

then:		; preds = %entry
	%calltmp = call double @foo()
	br label %ifcont

else:		; preds = %entry
	%calltmp1 = call double @bar()
	br label %ifcont

ifcont:		; preds = %else, %then
	%iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
	ret double %iftmp
}
</pre>
</div>

<p>To visualize the control flow graph, you can use a nifty feature of the LLVM
'<a href="http://llvm.org/cmds/opt.html">opt</a>' tool.  If you put this LLVM IR
into "t.ll" and run "<tt>llvm-as &lt; t.ll | opt -analyze -view-cfg</tt>", <a
href="http://www.llvm.org/docs/ProgrammersManual.html#ViewGraph">a window will
pop up</a> and you'll see this graph:</p>

<div style="text-align: center"><img src="http://www.llvm.org/docs/tutorial/LangImpl5-cfg.png" alt="Example CFG" width="423"
height="315"></div>

<p>Another way to get this is to call "<tt>function.viewCFG()</tt>" or
"<tt>function.viewCFGOnly()</tt>" (where F is a "<tt>llvm.core.Function</tt>")
either by inserting actual calls into the code and recompiling or by calling
these in the debugger.  LLVM has many nice features for visualizing various
graphs, but note that these are available only if your LLVM was built with
Graphviz support (accomplished by having Graphviz and Ghostview installed when
building LLVM).</p>

<p>Getting back to the generated code, it is fairly simple: the entry block
evaluates the conditional expression ("x" in our case here) and compares the
result to 0.0 with the
"<tt><a href="http://www.llvm.org/docs/LangRef.html#i_fcmp">fcmp</a> one</tt>"
instruction ('one' is "Ordered and Not Equal").  Based on the result of this
expression, the code jumps to either the "then" or "else" blocks, which contain
the expressions for the true/false cases.</p>

<p>Once the then/else blocks are finished executing, they both branch back to
the 'ifcont' block to execute the code that happens after the if/then/else.  In
this case the only thing left to do is to return to the caller of the function.
The question then becomes: how does the code know which expression to return?
</p>

<p>The answer to this question involves an important SSA operation: the
<a href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
operation</a>.  If you're not familiar with SSA, <a
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
article</a> is a good introduction and there are various other introductions to
it available on your favorite search engine.  The short version is that
"execution" of the Phi operation requires "remembering" which block control came
from.  The Phi operation takes on the value corresponding to the input control
block.  In this case, if control comes in from the "then" block, it gets the
value of "calltmp".  If control comes from the "else" block, it gets the value
of "calltmp1".</p>

<p>At this point, you are probably starting to think "Oh no! This means my
simple and elegant front-end will have to start generating SSA form in order to
use LLVM!".  Fortunately, this is not the case, and we strongly advise
<em>not</em> implementing an SSA construction algorithm in your front-end
unless there is an amazingly good reason to do so.  In practice, there are two
sorts of values that float around in code written for your average imperative
programming language that might need Phi nodes:</p>

<ol>
<li>Code that involves user variables: <tt>x = 1; x = x + 1;</tt></li>
<li>Values that are implicit in the structure of your AST, such as the Phi node
in this case.</li>
</ol>

<p>In <a href="PythonLangImpl7.html">Chapter 7</a> of this tutorial ("mutable
variables"), we'll talk about #1 in depth.  For now, just believe me that you
don't need SSA construction to handle this case.  For #2, you have the choice of
using the techniques that we will describe for #1, or you can insert Phi nodes
directly, if convenient.  In this case, it is really really easy to generate
the Phi node, so we choose to do it directly.</p>

<p>Okay, enough of the motivation and overview, lets generate code!</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="ifcodegen">Code Generation for
If/Then/Else</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>In order to generate code for this, we implement the <tt>Codegen</tt> method
for <tt>IfExpressionNode</tt>:</p>

<div class="doc_code">
<pre>
  def CodeGen(self):
    condition = self.condition.CodeGen()

    # Convert condition to a bool by comparing equal to 0.0.
    condition_bool = g_llvm_builder.fcmp(
        FCMP_ONE, condition, Constant.real(Type.double(), 0), 'ifcond')
</pre>
</div>

<p>This code is straightforward and similar to what we saw before.  We emit the
expression for the condition, then compare that value to zero to get a truth
value as a 1-bit (bool) value.</p>

<div class="doc_code">
<pre>
    function = g_llvm_builder.basic_block.function

    # Create blocks for the then and else cases. Insert the 'then' block at the
    # end of the function.
    then_block = function.append_basic_block('then')
    else_block = function.append_basic_block('else')
    merge_block = function.append_basic_block('ifcond')

    g_llvm_builder.cbranch(condition_bool, then_block, else_block)
</pre>
</div>

<p>This code creates the basic blocks that are related to the if/then/else
statement, and correspond directly to the blocks in the example above.  The
first line gets the current Function object that is being built.  It
gets this by asking the builder for the current BasicBlock, and asking that
block for its "parent" (the function it is currently embedded into).</p>

<p>Once it has that, it creates three block which are automatically inserted
into the end of the function. Once the blocks are created, we can emit the
conditional branch that chooses between them.  Note that creating new blocks
does not implicitly affect the Builder, so it is still inserting into the block
that the condition went into.</p>

<div class="doc_code">
<pre>
    # Emit then value.
    g_llvm_builder.position_at_end(then_block)
    then_value = self.then_branch.CodeGen()
    g_llvm_builder.branch(merge_block)

    # Codegen of 'Then' can change the current block; update then_block for the
    # PHI node.
    then_block = g_llvm_builder.basic_block
</pre>
</div>

<p>After the conditional branch is inserted, we move the builder to start
inserting into the "then" block.  Strictly speaking, this call moves the
insertion point to be at the end of the specified block.  However, since the
"then" block is empty, it also starts out by inserting at the beginning of the
block.  :)</p>

<p>Once the insertion point is set, we recursively codegen the "then" expression
from the AST.  To finish off the "then" block, we create an unconditional branch
to the merge block.  One interesting (and very important) aspect of the LLVM IR
is that it
<a href="http://www.llvm.org/docs/LangRef.html#functionstructure">requires all
basic blocks to be "terminated"</a> with a
<a href="http://www.llvm.org/docs/LangRef.html#terminators">control flow
instruction</a> such as return or branch.  This means that all control flow,
<em>including fallthroughs</em> must be made explicit in the LLVM IR.  If you
violate this rule, the verifier will emit an error.</p>

<p>The final line here is quite subtle, but is very important.  The basic issue
is that when we create the Phi node in the merge block, we need to set up the
block/value pairs that indicate how the Phi will work.  Importantly, the Phi
node expects to have an entry for each predecessor of the block in the CFG.  Why
then, are we getting the current block when we just set it to then_block 5 lines
above?  The problem is that the "Then" expression may actually itself change the
block that the Builder is emitting into if, for example, it contains a nested
"if/then/else" expression.  Because calling Codegen recursively could
arbitrarily change the notion of the current block, we are required to get an
up-to-date value for code that will set up the Phi node.</p>

<div class="doc_code">
<pre>
    # Emit else block.
    g_llvm_builder.position_at_end(else_block)
    else_value = self.else_branch.CodeGen()
    g_llvm_builder.branch(merge_block)

    # Codegen of 'Else' can change the current block, update else_block for the
    # PHI node.
    else_block = g_llvm_builder.basic_block
</pre>
</div>

<p>Code generation for the 'else' block is basically identical to codegen for
the 'then' block.  The only significant difference is the first line, which adds
the 'else' block to the function.  Recall previously that the 'else' block was
created, but not added to the function.  Now that the 'then' and 'else' blocks
are emitted, we can finish up with the merge code:</p>

<div class="doc_code">
<pre>
    # Emit merge block.
    g_llvm_builder.position_at_end(merge_block)
    phi = g_llvm_builder.phi(Type.double(), 'iftmp')
    phi.add_incoming(then_value, then_block)
    phi.add_incoming(else_value, else_block)

    return phi
</pre>
</div>

<p>The first line changes the insertion point so that newly created code will go
into the "merge" block.  Once that is done, we need to create the PHI node and
set up the block/value pairs for the PHI.</p>

<p>Finally, the CodeGen function returns the phi node as the value computed by
the if/then/else expression.  In our example above, this returned value will
feed into the code for the top-level function, which will create the return
instruction.</p>

<p>Overall, we now have the ability to execute conditional code in
Kaleidoscope.  With this extension, Kaleidoscope is a fairly complete language
that can calculate a wide variety of numeric functions.  Next up we'll add
another useful expression that is familiar from non-functional languages...</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="for">'for' Loop Expression</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Now that we know how to add basic control flow constructs to the language,
we have the tools to add more powerful things.  Lets add something more
aggressive, a 'for' expression:</p>

<div class="doc_code">
<pre>
 extern putchard(char)
 def printstar(n)
   for i = 1, i &lt; n, 1.0 in
     putchard(42)  # ascii 42 = '*'

 # print 100 '*' characters
 printstar(100)
</pre>
</div>

<p>This expression defines a new variable ("i" in this case) which iterates from
a starting value, while the condition ("i &lt; n" in this case) is true,
incrementing by an optional step value ("1.0" in this case).  If the step value
is omitted, it defaults to 1.0.  While the loop is true, it executes its
body expression.  Because we don't have anything better to return, we'll just
define the loop as always returning 0.0.  In the future when we have mutable
variables, it will get more useful.</p>

<p>As before, lets talk about the changes that we need to Kaleidoscope to
support this.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forlexer">Lexer Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>The lexer extensions are the same sort of thing as for if/then/else:</p>

<div class="doc_code">
<pre>
...

class ThenToken(object): pass
class ElseToken(object): pass
<b>class ForToken(object): pass
class InToken(object): pass</b>

...

def Tokenize(string):

      ...

      elif identifier == 'else':
        yield ElseToken()
      <b>elif identifier == 'for':
        yield ForToken()
      elif identifier == 'in':
        yield InToken()</b>
      else:
        yield IdentifierToken(identifier)
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forast">AST Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>The AST node is just as simple.  It basically boils down to capturing
the variable name and the constituent expressions in the node.</p>

<div class="doc_code">
<pre>
# Expression class for for/in.
class ForExpressionNode(ExpressionNode):

  def __init__(self, loop_variable, start, end, step, body):
    self.loop_variable = loop_variable
    self.start = start
    self.end = end
    self.step = step
    self.body = body

  def CodeGen(self):
    ...
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forparser">Parser Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>The parser code is also fairly standard.  The only interesting thing here is
handling of the optional step value.  The parser code handles it by checking to
see if the second comma is present.  If not, it sets the step value to null in
the AST node:</p>

<div class="doc_code">
<pre>
  # forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
  def ParseForExpr(self):
    self.Next()  # eat the for.

    if not isinstance(self.current, IdentifierToken):
      raise RuntimeError('Expected identifier after for.')

    loop_variable = self.current.name
    self.Next()  # eat the identifier.

    if self.current != CharacterToken('='):
      raise RuntimeError('Expected "=" after for variable.')
    self.Next()  # eat the '='.

    start = self.ParseExpression()

    if self.current != CharacterToken(','):
      raise RuntimeError('Expected "," after for start value.')
    self.Next()  # eat the ','.

    end = self.ParseExpression()

    # The step value is optional.
    if self.current == CharacterToken(','):
      self.Next()  # eat the ','.
      step = self.ParseExpression()
    else:
      step = None

    if not isinstance(self.current, InToken):
      raise RuntimeError('Expected "in" after for variable specification.')
    self.Next()  # eat 'in'.

    body = self.ParseExpression()

    return ForExpressionNode(loop_variable, start, end, step, body)
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forir">LLVM IR for
the 'for' Loop</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>Now we get to the good part: the LLVM IR we want to generate for this thing.
With the simple example above, we get this LLVM IR (note that this dump is
generated with optimizations disabled for clarity):
</p>

<div class="doc_code">
<pre>
declare double @putchard(double)

define double @printstar(double %n) {
entry:
        ; initial value = 1.0 (inlined into phi)
	br label %loop

loop:		; preds = %loop, %entry
	%i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ]
        ; body
	%calltmp = call double @putchard(double 4.200000e+01)
        ; increment
	%nextvar = fadd double %i, 1.000000e+00

        ; termination test
	%cmptmp = fcmp ult double %i, %n
	%booltmp = uitofp i1 %cmptmp to double
	%loopcond = fcmp one double %booltmp, 0.000000e+00
	br i1 %loopcond, label %loop, label %afterloop

afterloop:		; preds = %loop
        ; loop always returns 0.0
	ret double 0.000000e+00
}
</pre>
</div>

<p>This loop contains all the same constructs we saw before: a phi node, several
expressions, and some basic blocks.  Lets see how this fits together.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forcodegen">Code Generation for
the 'for' Loop</a></div>
<!-- ======================================================================= -->

<div class="doc_text">

<p>The first part of Codegen is very simple: we just output the start expression
for the loop value:</p>

<div class="doc_code">
<pre>
  def CodeGen(self):
    # Emit the start code first, without 'variable' in scope.
    start_value = self.start.CodeGen()
</pre>
</div>

<p>With this out of the way, the next step is to set up the LLVM basic block
for the start of the loop body.  In the case above, the whole loop body is one
block, but remember that the body code itself could consist of multiple blocks
(e.g. if it contains an if/then/else or a for/in expression).</p>

<div class="doc_code">
<pre>
    # Make the new basic block for the loop header, inserting after current
    # block.
    function = g_llvm_builder.basic_block.function
    pre_header_block = g_llvm_builder.basic_block
    loop_block = function.append_basic_block('loop')

    # Insert an explicit fallthrough from the current block to the loop_block.
    g_llvm_builder.branch(loop_block)
</pre>
</div>

<p>This code is similar to what we saw for if/then/else.  Because we will need
it to create the Phi node, we remember the block that falls through into the
loop.  Once we have that, we create the actual block that starts the loop and
create an unconditional branch for the fall-through between the two blocks.</p>

<div class="doc_code">
<pre>
    # Start insertion in loop_block.
    g_llvm_builder.position_at_end(loop_block);

    # Start the PHI node with an entry for start.
    variable_phi = g_llvm_builder.phi(Type.double(), self.loop_variable)
    variable_phi.add_incoming(start_value, pre_header_block)
</pre>
</div>

<p>Now that the "pre_header_block" for the loop is set up, we switch to emitting
code for the loop body.  To begin with, we move the insertion point and create
the PHI node for the loop induction variable.  Since we already know the
incoming value for the starting value, we add it to the Phi node.  Note that the
Phi will eventually get a second value for the backedge, but we can't set it up
yet (because it doesn't exist!).</p>

<div class="doc_code">
<pre>
    # Within the loop, the variable is defined equal to the PHI node.  If it
    # shadows an existing variable, we have to restore it, so save it now.
    old_value = g_named_values.get(self.loop_variable, None)
    g_named_values[self.loop_variable] = variable_phi

    # Emit the body of the loop.  This, like any other expr, can change the
    # current BB.  Note that we ignore the value computed by the body.
    self.body.CodeGen()
</pre>
</div>

<p>Now the code starts to get more interesting.  Our 'for' loop introduces a new
variable to the symbol table.  This means that our symbol table can now contain
either function arguments or loop variables.  To handle this, before we codegen
the body of the loop, we add the loop variable as the current value for its
name.  Note that it is possible that there is a variable of the same name in the
outer scope.  It would be easy to make this an error (emit an error and return
null if there is already an entry for VarName) but we choose to allow shadowing
of variables.  In order to handle this correctly, we remember the Value that
we are potentially shadowing in <tt>old_value</tt> (which will be None if there
is no shadowed variable).</p>

<p>Once the loop variable is set into the symbol table, the code recursively
codegen's the body.  This allows the body to use the loop variable: any
references to it will naturally find it in the symbol table.</p>

<div class="doc_code">
<pre>
    # Emit the step value.
    if self.step:
      step_value = self.step.CodeGen()
    else:
      # If not specified, use 1.0.
      step_value = Constant.real(Type.double(), 1)

    next_value = g_llvm_builder.fadd(variable_phi, step_value, 'next')
</pre>
</div>

<p>Now that the body is emitted, we compute the next value of the iteration
variable by adding the step value, or 1.0 if it isn't present.
<tt>next_value</tt> will be the value of the loop variable on the next iteration
of the loop.</p>

<div class="doc_code">
<pre>
    # Compute the end condition and convert it to a bool by comparing to 0.0.
    end_condition = self.end.CodeGen()
    end_condition_bool = g_llvm_builder.fcmp(
        FCMP_ONE, end_condition, Constant.real(Type.double(), 0), 'loopcond')
</pre>
</div>

<p>Finally, we evaluate the exit value of the loop, to determine whether the
loop should exit.  This mirrors the condition evaluation for the if/then/else
statement.</p>

<div class="doc_code">
<pre>
    # Create the "after loop" block and insert it.
    loop_end_block = g_llvm_builder.basic_block
    after_block = function.append_basic_block('afterloop')

    # Insert the conditional branch into the end of loop_end_block.
    g_llvm_builder.cbranch(end_condition_bool, loop_block, after_block)

    # Any new code will be inserted in after_block.
    g_llvm_builder.position_at_end(after_block)
</pre>
</div>

<p>With the code for the body of the loop complete, we just need to finish up
the control flow for it.  This code remembers the end block (for the phi node),
then creates the block for the loop exit ("afterloop").  Based on the value of
the exit condition, it creates a conditional branch that chooses between
executing the loop again and exiting the loop.  Any future code is emitted in
the "afterloop" block, so it sets the insertion position to it.</p>

<div class="doc_code">
<pre>
    # Add a new entry to the PHI node for the backedge.
    variable_phi.add_incoming(next_value, loop_end_block)

    # Restore the unshadowed variable.
    if old_value:
      g_named_values[self.loop_variable] = old_value
    else:
      del g_named_values[self.loop_variable]

    # for expr always returns 0.0.
    return Constant.real(Type.double(), 0)
</pre>
</div>

<p>The final code handles various cleanups: now that we have the "next_value",
we can add the incoming value to the loop PHI node.  After that, we remove the
loop variable from the symbol table, so that it isn't in scope after the for
loop.  Finally, code generation of the for loop always returns 0.0, so that is
what we return from <tt>ForExpressionNode::CodeGen</tt>.</p>

<p>With this, we conclude the "adding control flow to Kaleidoscope" chapter of
the tutorial.  In this chapter we added two control flow constructs, and used
them to motivate a couple of aspects of the LLVM IR that are important for
front-end implementors to know.  In the next chapter of our saga, we will get a
bit crazier and add <a href="PythonLangImpl6.html">user-defined operators</a> to
our poor innocent language.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="code">Full Code Listing</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
Here is the complete code listing for our running example, enhanced with the
if/then/else and for expressions:</p>

<div class="doc_code">
<pre>
#!/usr/bin/env python

import re
from llvm.core import Module, Constant, Type, Function, Builder
from llvm.ee import ExecutionEngine, TargetData
from llvm.passes import FunctionPassManager

from llvm.core import FCMP_ULT, FCMP_ONE
from llvm.passes import (PASS_INSTRUCTION_COMBINING,
                         PASS_REASSOCIATE,
                         PASS_GVN,
                         PASS_CFG_SIMPLIFICATION)

################################################################################
## Globals
################################################################################

# The LLVM module, which holds all the IR code.
g_llvm_module = Module.new('my cool jit')

# The LLVM instruction builder. Created whenever a new function is entered.
g_llvm_builder = None

# A dictionary that keeps track of which values are defined in the current scope
# and what their LLVM representation is.
g_named_values = {}

# The function optimization passes manager.
g_llvm_pass_manager = FunctionPassManager.new(g_llvm_module)

# The LLVM execution engine.
g_llvm_executor = ExecutionEngine.new(g_llvm_module)

################################################################################
## Lexer
################################################################################

# The lexer yields one of these types for each token.
class EOFToken(object): pass
class DefToken(object): pass
class ExternToken(object): pass
class IfToken(object): pass
class ThenToken(object): pass
class ElseToken(object): pass
class ForToken(object): pass
class InToken(object): pass

class IdentifierToken(object):
  def __init__(self, name): self.name = name

class NumberToken(object):
  def __init__(self, value): self.value = value

class CharacterToken(object):
  def __init__(self, char): self.char = char
  def __eq__(self, other):
    return isinstance(other, CharacterToken) and self.char == other.char
  def __ne__(self, other): return not self == other

# Regular expressions that tokens and comments of our language.
REGEX_NUMBER = re.compile('[0-9]+(?:\.[0-9]+)?')
REGEX_IDENTIFIER = re.compile('[a-zA-Z][a-zA-Z0-9]*')
REGEX_COMMENT = re.compile('#.*')

def Tokenize(string):
  while string:
    # Skip whitespace.
    if string[0].isspace():
      string = string[1:]
      continue

    # Run regexes.
    comment_match = REGEX_COMMENT.match(string)
    number_match = REGEX_NUMBER.match(string)
    identifier_match = REGEX_IDENTIFIER.match(string)

    # Check if any of the regexes matched and yield the appropriate result.
    if comment_match:
      comment = comment_match.group(0)
      string = string[len(comment):]
    elif number_match:
      number = number_match.group(0)
      yield NumberToken(float(number))
      string = string[len(number):]
    elif identifier_match:
      identifier = identifier_match.group(0)
      # Check if we matched a keyword.
      if identifier == 'def':
        yield DefToken()
      elif identifier == 'extern':
        yield ExternToken()
      elif identifier == 'if':
        yield IfToken()
      elif identifier == 'then':
        yield ThenToken()
      elif identifier == 'else':
        yield ElseToken()
      elif identifier == 'for':
        yield ForToken()
      elif identifier == 'in':
        yield InToken()
      else:
        yield IdentifierToken(identifier)
      string = string[len(identifier):]
    else:
      # Yield the ASCII value of the unknown character.
      yield CharacterToken(string[0])
      string = string[1:]

  yield EOFToken()

################################################################################
## Abstract Syntax Tree (aka Parse Tree)
################################################################################

# Base class for all expression nodes.
class ExpressionNode(object):
  pass

# Expression class for numeric literals like "1.0".
class NumberExpressionNode(ExpressionNode):

  def __init__(self, value):
    self.value = value

  def CodeGen(self):
    return Constant.real(Type.double(), self.value)

# Expression class for referencing a variable, like "a".
class VariableExpressionNode(ExpressionNode):

  def __init__(self, name):
    self.name = name

  def CodeGen(self):
    if self.name in g_named_values:
      return g_named_values[self.name]
    else:
      raise RuntimeError('Unknown variable name: ' + self.name)

# Expression class for a binary operator.
class BinaryOperatorExpressionNode(ExpressionNode):

  def __init__(self, operator, left, right):
    self.operator = operator
    self.left = left
    self.right = right

  def CodeGen(self):
    left = self.left.CodeGen()
    right = self.right.CodeGen()

    if self.operator == '+':
      return g_llvm_builder.fadd(left, right, 'addtmp')
    elif self.operator == '-':
      return g_llvm_builder.fsub(left, right, 'subtmp')
    elif self.operator == '*':
      return g_llvm_builder.fmul(left, right, 'multmp')
    elif self.operator == '&lt;':
      result = g_llvm_builder.fcmp(FCMP_ULT, left, right, 'cmptmp')
      # Convert bool 0 or 1 to double 0.0 or 1.0.
      return g_llvm_builder.uitofp(result, Type.double(), 'booltmp')
    else:
      raise RuntimeError('Unknown binary operator.')

# Expression class for function calls.
class CallExpressionNode(ExpressionNode):

  def __init__(self, callee, args):
    self.callee = callee
    self.args = args

  def CodeGen(self):
    # Look up the name in the global module table.
    callee = g_llvm_module.get_function_named(self.callee)

    # Check for argument mismatch error.
    if len(callee.args) != len(self.args):
      raise RuntimeError('Incorrect number of arguments passed.')

    arg_values = [i.CodeGen() for i in self.args]

    return g_llvm_builder.call(callee, arg_values, 'calltmp')

# Expression class for if/then/else.
class IfExpressionNode(ExpressionNode):

  def __init__(self, condition, then_branch, else_branch):
    self.condition = condition
    self.then_branch = then_branch
    self.else_branch = else_branch

  def CodeGen(self):
    condition = self.condition.CodeGen()

    # Convert condition to a bool by comparing equal to 0.0.
    condition_bool = g_llvm_builder.fcmp(
        FCMP_ONE, condition, Constant.real(Type.double(), 0), 'ifcond')

    function = g_llvm_builder.basic_block.function

    # Create blocks for the then and else cases. Insert the 'then' block at the
    # end of the function.
    then_block = function.append_basic_block('then')
    else_block = function.append_basic_block('else')
    merge_block = function.append_basic_block('ifcond')

    g_llvm_builder.cbranch(condition_bool, then_block, else_block)

    # Emit then value.
    g_llvm_builder.position_at_end(then_block)
    then_value = self.then_branch.CodeGen()
    g_llvm_builder.branch(merge_block)

    # Codegen of 'Then' can change the current block; update then_block for the
    # PHI node.
    then_block = g_llvm_builder.basic_block

    # Emit else block.
    g_llvm_builder.position_at_end(else_block)
    else_value = self.else_branch.CodeGen()
    g_llvm_builder.branch(merge_block)

    # Codegen of 'Else' can change the current block, update else_block for the
    # PHI node.
    else_block = g_llvm_builder.basic_block

    # Emit merge block.
    g_llvm_builder.position_at_end(merge_block)
    phi = g_llvm_builder.phi(Type.double(), 'iftmp')
    phi.add_incoming(then_value, then_block)
    phi.add_incoming(else_value, else_block)

    return phi

# Expression class for for/in.
class ForExpressionNode(ExpressionNode):

  def __init__(self, loop_variable, start, end, step, body):
    self.loop_variable = loop_variable
    self.start = start
    self.end = end
    self.step = step
    self.body = body

  def CodeGen(self):
    # Output this as:
    #   ...
    #   start = startexpr
    #   goto loop
    # loop:
    #   variable = phi [start, loopheader], [nextvariable, loopend]
    #   ...
    #   bodyexpr
    #   ...
    # loopend:
    #   step = stepexpr
    #   nextvariable = variable + step
    #   endcond = endexpr
    #   br endcond, loop, endloop
    # outloop:

    # Emit the start code first, without 'variable' in scope.
    start_value = self.start.CodeGen()

    # Make the new basic block for the loop header, inserting after current
    # block.
    function = g_llvm_builder.basic_block.function
    pre_header_block = g_llvm_builder.basic_block
    loop_block = function.append_basic_block('loop')

    # Insert an explicit fallthrough from the current block to the loop_block.
    g_llvm_builder.branch(loop_block)

    # Start insertion in loop_block.
    g_llvm_builder.position_at_end(loop_block)

    # Start the PHI node with an entry for start.
    variable_phi = g_llvm_builder.phi(Type.double(), self.loop_variable)
    variable_phi.add_incoming(start_value, pre_header_block)

    # Within the loop, the variable is defined equal to the PHI node.  If it
    # shadows an existing variable, we have to restore it, so save it now.
    old_value = g_named_values.get(self.loop_variable, None)
    g_named_values[self.loop_variable] = variable_phi

    # Emit the body of the loop.  This, like any other expr, can change the
    # current BB.  Note that we ignore the value computed by the body.
    self.body.CodeGen()

    # Emit the step value.
    if self.step:
      step_value = self.step.CodeGen()
    else:
      # If not specified, use 1.0.
      step_value = Constant.real(Type.double(), 1)

    next_value = g_llvm_builder.fadd(variable_phi, step_value, 'next')

    # Compute the end condition and convert it to a bool by comparing to 0.0.
    end_condition = self.end.CodeGen()
    end_condition_bool = g_llvm_builder.fcmp(
        FCMP_ONE, end_condition, Constant.real(Type.double(), 0), 'loopcond')

    # Create the "after loop" block and insert it.
    loop_end_block = g_llvm_builder.basic_block
    after_block = function.append_basic_block('afterloop')

    # Insert the conditional branch into the end of loop_end_block.
    g_llvm_builder.cbranch(end_condition_bool, loop_block, after_block)

    # Any new code will be inserted in after_block.
    g_llvm_builder.position_at_end(after_block)

    # Add a new entry to the PHI node for the backedge.
    variable_phi.add_incoming(next_value, loop_end_block)

    # Restore the unshadowed variable.
    if old_value:
      g_named_values[self.loop_variable] = old_value
    else:
      del g_named_values[self.loop_variable]

    # for expr always returns 0.0.
    return Constant.real(Type.double(), 0)

# This class represents the "prototype" for a function, which captures its name,
# and its argument names (thus implicitly the number of arguments the function
# takes).
class PrototypeNode(object):

  def __init__(self, name, args):
    self.name = name
    self.args = args

  def CodeGen(self):
    # Make the function type, eg. double(double,double).
    funct_type = Type.function(
      Type.double(), [Type.double()] * len(self.args), False)

    function = Function.new(g_llvm_module, funct_type, self.name)

    # If the name conflicted, there was already something with the same name.
    # If it has a body, don't allow redefinition or reextern.
    if function.name != self.name:
      function.delete()
      function = g_llvm_module.get_function_named(self.name)

      # If the function already has a body, reject this.
      if not function.is_declaration:
        raise RuntimeError('Redefinition of function.')

      # If the function took a different number of args, reject.
      if len(function.args) != len(self.args):
        raise RuntimeError('Redeclaration of a function with different number '
                           'of args.')

    # Set names for all arguments and add them to the variables symbol table.
    for arg, arg_name in zip(function.args, self.args):
      arg.name = arg_name
      # Add arguments to variable symbol table.
      g_named_values[arg_name] = arg

    return function

# This class represents a function definition itself.
class FunctionNode(object):

  def __init__(self, prototype, body):
    self.prototype = prototype
    self.body = body

  def CodeGen(self):
    # Clear scope.
    g_named_values.clear()

    # Create a function object.
    function = self.prototype.CodeGen()

    # Create a new basic block to start insertion into.
    block = function.append_basic_block('entry')
    global g_llvm_builder
    g_llvm_builder = Builder.new(block)

    # Finish off the function.
    try:
      return_value = self.body.CodeGen()
      g_llvm_builder.ret(return_value)

      # Validate the generated code, checking for consistency.
      function.verify()

      # Optimize the function.
      g_llvm_pass_manager.run(function)
    except:
      function.delete()
      raise

    return function


################################################################################
## Parser
################################################################################

class Parser(object):

  def __init__(self, tokens, binop_precedence):
    self.tokens = tokens
    self.binop_precedence = binop_precedence
    self.Next()

  # Provide a simple token buffer. Parser.current is the current token the
  # parser is looking at. Parser.Next() reads another token from the lexer and
  # updates Parser.current with its results.
  def Next(self):
    self.current = self.tokens.next()

  # Gets the precedence of the current token, or -1 if the token is not a binary
  # operator.
  def GetCurrentTokenPrecedence(self):
    if isinstance(self.current, CharacterToken):
      return self.binop_precedence.get(self.current.char, -1)
    else:
      return -1

  # identifierexpr ::= identifier | identifier '(' expression* ')'
  def ParseIdentifierExpr(self):
    identifier_name = self.current.name
    self.Next()  # eat identifier.

    if self.current != CharacterToken('('):  # Simple variable reference.
      return VariableExpressionNode(identifier_name)

    # Call.
    self.Next()  # eat '('.
    args = []
    if self.current != CharacterToken(')'):
      while True:
        args.append(self.ParseExpression())
        if self.current == CharacterToken(')'):
          break
        elif self.current != CharacterToken(','):
          raise RuntimeError('Expected ")" or "," in argument list.')
        self.Next()

    self.Next()  # eat ')'.
    return CallExpressionNode(identifier_name, args)

  # numberexpr ::= number
  def ParseNumberExpr(self):
    result = NumberExpressionNode(self.current.value)
    self.Next()  # consume the number.
    return result

  # parenexpr ::= '(' expression ')'
  def ParseParenExpr(self):
    self.Next()  # eat '('.

    contents = self.ParseExpression()

    if self.current != CharacterToken(')'):
      raise RuntimeError('Expected ")".')
    self.Next()  # eat ')'.

    return contents

  # ifexpr ::= 'if' expression 'then' expression 'else' expression
  def ParseIfExpr(self):
    self.Next()  # eat the if.

    # condition.
    condition = self.ParseExpression()

    if not isinstance(self.current, ThenToken):
      raise RuntimeError('Expected "then".')
    self.Next()  # eat the then.

    then_branch = self.ParseExpression()

    if not isinstance(self.current, ElseToken):
      raise RuntimeError('Expected "else".')
    self.Next()  # eat the else.

    else_branch = self.ParseExpression()

    return IfExpressionNode(condition, then_branch, else_branch)

  # forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
  def ParseForExpr(self):
    self.Next()  # eat the for.

    if not isinstance(self.current, IdentifierToken):
      raise RuntimeError('Expected identifier after for.')

    loop_variable = self.current.name
    self.Next()  # eat the identifier.

    if self.current != CharacterToken('='):
      raise RuntimeError('Expected "=" after for variable.')
    self.Next()  # eat the '='.

    start = self.ParseExpression()

    if self.current != CharacterToken(','):
      raise RuntimeError('Expected "," after for start value.')
    self.Next()  # eat the ','.

    end = self.ParseExpression()

    # The step value is optional.
    if self.current == CharacterToken(','):
      self.Next()  # eat the ','.
      step = self.ParseExpression()
    else:
      step = None

    if not isinstance(self.current, InToken):
      raise RuntimeError('Expected "in" after for variable specification.')
    self.Next()  # eat 'in'.

    body = self.ParseExpression()

    return ForExpressionNode(loop_variable, start, end, step, body)

  # primary ::= identifierexpr | numberexpr | parenexpr | ifexpr | forexpr
  def ParsePrimary(self):
    if isinstance(self.current, IdentifierToken):
      return self.ParseIdentifierExpr()
    elif isinstance(self.current, NumberToken):
      return self.ParseNumberExpr()
    elif isinstance(self.current, IfToken):
      return self.ParseIfExpr()
    elif isinstance(self.current, ForToken):
      return self.ParseForExpr()
    elif self.current == CharacterToken('('):
      return self.ParseParenExpr()
    else:
      raise RuntimeError('Unknown token when expecting an expression.')

  # binoprhs ::= (operator primary)*
  def ParseBinOpRHS(self, left, left_precedence):
    # If this is a binary operator, find its precedence.
    while True:
      precedence = self.GetCurrentTokenPrecedence()

      # If this is a binary operator that binds at least as tightly as the
      # current one, consume it; otherwise we are done.
      if precedence &lt; left_precedence:
        return left

      binary_operator = self.current.char
      self.Next()  # eat the operator.

      # Parse the primary expression after the binary operator.
      right = self.ParsePrimary()

      # If binary_operator binds less tightly with right than the operator after
      # right, let the pending operator take right as its left.
      next_precedence = self.GetCurrentTokenPrecedence()
      if precedence &lt; next_precedence:
        right = self.ParseBinOpRHS(right, precedence + 1)

      # Merge left/right.
      left = BinaryOperatorExpressionNode(binary_operator, left, right)

  # expression ::= primary binoprhs
  def ParseExpression(self):
    left = self.ParsePrimary()
    return self.ParseBinOpRHS(left, 0)

  # prototype ::= id '(' id* ')'
  def ParsePrototype(self):
    if not isinstance(self.current, IdentifierToken):
      raise RuntimeError('Expected function name in prototype.')

    function_name = self.current.name
    self.Next()  # eat function name.

    if self.current != CharacterToken('('):
      raise RuntimeError('Expected "(" in prototype.')
    self.Next()  # eat '('.

    arg_names = []
    while isinstance(self.current, IdentifierToken):
      arg_names.append(self.current.name)
      self.Next()

    if self.current != CharacterToken(')'):
      raise RuntimeError('Expected ")" in prototype.')

    # Success.
    self.Next()  # eat ')'.

    return PrototypeNode(function_name, arg_names)

  # definition ::= 'def' prototype expression
  def ParseDefinition(self):
    self.Next()  # eat def.
    proto = self.ParsePrototype()
    body = self.ParseExpression()
    return FunctionNode(proto, body)

  # toplevelexpr ::= expression
  def ParseTopLevelExpr(self):
    proto = PrototypeNode('', [])
    return FunctionNode(proto, self.ParseExpression())

  # external ::= 'extern' prototype
  def ParseExtern(self):
    self.Next()  # eat extern.
    return self.ParsePrototype()

  # Top-Level parsing
  def HandleDefinition(self):
    self.Handle(self.ParseDefinition, 'Read a function definition:')

  def HandleExtern(self):
    self.Handle(self.ParseExtern, 'Read an extern:')

  def HandleTopLevelExpression(self):
    try:
      function = self.ParseTopLevelExpr().CodeGen()
      result = g_llvm_executor.run_function(function, [])
      print 'Evaluated to:', result.as_real(Type.double())
    except Exception, e:
      print 'Error:', e
      try:
        self.Next() # Skip for error recovery.
      except:
        pass

  def Handle(self, function, message):
    try:
      print message, function().CodeGen()
    except Exception, e:
      print 'Error:', e
      try:
        self.Next() # Skip for error recovery.
      except:
        pass

################################################################################
## Main driver code.
################################################################################

def main():
  # Set up the optimizer pipeline. Start with registering info about how the
  # target lays out data structures.
  g_llvm_pass_manager.add(g_llvm_executor.target_data)
  # Do simple "peephole" optimizations and bit-twiddling optzns.
  g_llvm_pass_manager.add(PASS_INSTRUCTION_COMBINING)
  # Reassociate expressions.
  g_llvm_pass_manager.add(PASS_REASSOCIATE)
  # Eliminate Common SubExpressions.
  g_llvm_pass_manager.add(PASS_GVN)
  # Simplify the control flow graph (deleting unreachable blocks, etc).
  g_llvm_pass_manager.add(PASS_CFG_SIMPLIFICATION)

  g_llvm_pass_manager.initialize()

  # Install standard binary operators.
  # 1 is lowest possible precedence. 40 is the highest.
  operator_precedence = {
    '&lt;': 10,
    '+': 20,
    '-': 20,
    '*': 40
  }

  # Run the main "interpreter loop".
  while True:
    print 'ready&gt;',
    try:
      raw = raw_input()
    except KeyboardInterrupt:
      break

    parser = Parser(Tokenize(raw), operator_precedence)
    while True:
      # top ::= definition | external | expression | EOF
      if isinstance(parser.current, EOFToken):
        break
      if isinstance(parser.current, DefToken):
        parser.HandleDefinition()
      elif isinstance(parser.current, ExternToken):
        parser.HandleExtern()
      else:
        parser.HandleTopLevelExpression()

  # Print out all of the generated code.
  print '\n', g_llvm_module

if __name__ == '__main__':
  main()
</pre>
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<a href="PythonLangImpl6.html">Next: Extending the language: user-defined operators</a>
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